As engines (internal combustion engines), an engine which can perform a reduced-cylinder operation, in which a specific cylinder is maintained in a resting state, has been proposed as a measure for reducing fuel consumption (e.g., see Patent Document 1). In such an engine, injection of fuel and operation of intake or exhaust valves in the specific cylinder are stopped and the other operating cylinders are operated as a high-load operation, thereby enhancing a fuel consumption rate.
However, in the resting cylinder, in which injection of fuel and operation of intake or exhaust valves are stopped, a pressure in a combustion chamber thereof is decreased when a piston therein descends. That is, there is a problem that lubricant oil is gathered inside a combustion chamber of a cylinder which is maintained in the resting state during a long time.
Thus, if the cylinder is switched from a reduced-cylinder operation into an all-cylinder operation in a state where the lubricant oil is gathered inside the combustion chamber thereof, the lubricant oil is discharged together with an exhaust gas upon restarting of the all-cylinder operation. In this case, white smoke is likely to significantly occur even if a catalyst is equipped in an exhaust gas passage of the engine.
Also, in the reduced-cylinder operation in which only a specific cylinder is maintained in the resting state, there is a problem in that the reduced-cylinder operation can only be performed by a combination of cylinders having equal intervals or a good cylinder balance. For example, examples of reduced-cylinder operations of an 8-cylinder engine include a 6-cylinder operation and a 4-cylinder operation, and examples of reduced-cylinder operations of a 6-cylinder engine include a 3-cylinder operation and a 2-cylinder operation.
That is, in the reduced-cylinder operation in which only a specific cylinder is maintained in the resting state, a 7-cylinder operation or 5-cylinder operation in the 8-cylinder engine and a 5-cylinder operation in the 6-cylinder engine cannot be performed, thereby causing an output range from the engine during the reduced-cylinder operation to be discontinuous. Therefore, there is a problem that an operation range of the reduced-cylinder operation is narrow. Further, in the case of engines, in which a cylinder number is an odd number, such as a 5-cylinder engine or a 3-cylinder engine, no combination of cylinders having equal intervals or a good cylinder balance for the reduced-cylinder operation in which only a specific cylinder is maintained in the resting state exists and hence it is not possible to perform the reduced-cylinder operation thereon.